The present invention relates to a method for manufacturing a hybrid integrated circuit in which circuit elements such as a thick film resistor are formed on a substrate constituted by sintering, e.g., an alumina green sheet and, more particularly, to a method for manufacturing a hybrid integrated circuit in which a terminal conductor is formed on a substrate surface so as to connect, e.g. a thick film resistor.
A substrate constituting a hybrid integrated circuit is constituted by printing a refractory conductor material such as tungsten (W) and molybdenum (Mo) to form a wiring conductor and baking the resultant substrate on which the conductor material if printed in a reducing atmosphere at a temperature of about 1,600.degree. C. Thereafter, circuit elements such as a thick film resistor and a flip chip are formed on the baked alumina substrate.
Conventionally, in order to form, e.g., the thick film resistor on the alumina substrate, a silver-based conductor such as an Ag, Ag--Pt, or Ag--Pd conductor is formed as the terminal conductor on a printed conductor material constituting the wiring conductor, and then a thick film resistor layer is formed such that it is electrically connected to the terminal conductor.
The above substrate is sometimes constituted by a multilayer substrate obtained by laminating a plurality of substrates, and forming wiring layers made of the above-mentioned refractory conductor material therebetween. In such a multilayer substrate, conductor materials serving as conductor wiring formed inside the substrate are electrically connected to flip chips and the like in a manner as shown in FIG. 1,
In FIG. 1, multilayer substrate 11 is constituted by laminating a plurality of substrates 111, 112, . . . , and wiring conductor layer 12 is formed therebetween, as required. In order to electrically connect layer 12 to the surface of substrate 11, through-hole 13 is formed in uppermost substrate 111, and conductor 14 is filled with nickel, gold, or the like, in hole 13 so that the surface of conductor 14 is substantially flattened to obtain the same surface level as that of substrate 11. For example, thick film conductor 15 is formed on the surface of substrate 11 by printing, and layer 12 and conductor 15 are electrically connected to each other by conductor 14. Conductor 15 serves as a terminal conductor, and, a flip chip for example is mounted thereon by soldering (Pb--Sn), and the flip chip and layer 12 are electrically connected to each other. In addition, a thick film resistor (not shown) is formed on substrate 11 as required so as to partially overlap the above thick film conductor.
In order to form the thick film resistor on substrate 11, conductor 15, serving as a terminal conductor and a resistor material layer for forming the thick film resistor, are baked in air at about 850.degree. C. To prevent conductor 15 form being oxidized during the baking, precautions must be taken. For example, an Au film is formed on the surface of the conductor 15. The use of Au film results in high cost.
Furthermore, in the case of mounting a flip chip and the like on substrate 11, hole 13, formed in uppermost substrate 111 of substrate 11 is filled with a conductor. The surface of this conductor is smoothered so that conductor 15 which is formed on substrate 111, is continuous and has uniform thickness. In the case of forming the flip chip of soldering material on the surface of the conductor material constituting conductor 15, Ni- and Au-plated layers are formed between the solder and the conductor material. However, since Ni has a relatively strong tendency to become ionized, the surface of the conductor material must be activated by a catalyst when it is plated.